That is funny, I was thinking about that just last week. I was swimming at the local pool and was doing some faster laps and noticed that my mouth half filled with water sometimes on my breaths. The wierd thing was that it wasn't a problem. I was only getting air in my lungs. so It must of had to do with my head position. At the beginning of the summer as I started swimmiing again, I would get water in my airholes a lot more, but as I have swam more it has decreased. I used to get water quite a bit in my sinuses but that is rare now.
This last saturday on the other hand I was doing a mile swim with a local tri club up in Horseshoe Lake at 9000'. That makes for some cold water.
Somehow I managed to get full mouthful of water go down the wrong pipe. Not a fun feeling in the middle of the lake.